I can honestly say I don't know right where it is - but you ought to be able to find it...
Find the starter solenoid. Likely under one of the side covers, one large wire leads up to the battery, the other the starter. On the post that has the lead to the battery there is at least one other wire, just follow that, you should come upon the master fuse inline with the wire that leads to the fuse block.
I was just looking at that cartoon wiring diagram, I just put it up as a gag, but it's pretty good. It shows the master fuse just fine. Old early 70's Honda fours pretty much follow that convention.